This invention relates to a chemical process for removing sulfur dioxide from a sulfur dioxide-containing gas stream; more specifically, the process comprises contacting the gas with an aqueous sulfite solution containing sulfite anion, magnesium cation, and one or more other cations, converting sulfur dioxide and sulfite to bisulfite, and then regenerating the solution with magnesium oxide or hydroxide, separating magnesium sulfite from the regenerated solution, and recycling the aqueous sulfite solution.
The removal of sulfur dioxide from various gas streams has been extensively investigated. In spite of intensive efforts, more effective methods are still needed. This is especially so when the gas stream is waste gas, such as flue gas from combustion of a sulfur-containing fuel, e.g., coal, in a commercial steam power plant. Such flue gas is produced in large volumes at near atmospheric pressure and generally contains only about 0.04 to 0.5 percent (400 to 5000 ppm) sulfur dioxide. Simply handling and treating large gas volumes requires a substantial capital investment, and the expense is compounded when the treatment is inefficient in removing sulfur dioxide. An impediment to development of more effective means of sulfur dioxide removal has been the limited value of the by-products recoverable in the processes commonly in use today.
Since enactment of the Clean Air Act of 1971 and recognition that human exposure to sulfur dioxide levels of the order of 0.1 ppm causes adverse health effects, renewed efforts have been made to find a technically and economically feasible means to remove sulfur dioxide from sulphur dioxide-containing gas streams, such as flue gas.
Methods developed for removing sulfur dioxide from gas streams are described in the prior art, e.g., Riesenfeld and Kohl, "Gas Purification," Second Ed., Gulf Publishing Co., Houston, Texas, 1974, pp 262-341. Removal processes can be classified in various ways. For example, some of the processes trap and convert the gaseous sulfur dioxide to a solid, which itself is a waste material and requires disposal. Among such processes is the reaction of the sulfur dioxide with aqueous lime or limestone slurries, producing calcium sulfite or sulfate. Sorption in such nonregenerable slurries is the most common method now used for flue gas desulfurization.
Regenerable processes, on the other hand, permit regeneration of the contacting medium and, in some cases, optional recovery and beneficial use of the sulfur dioxide as well. Regenerable processes have obvious ecological advantages.
Many of the sulfur dioxide processes are based upon acid-base reactions. The acid gas, SO.sub.2, may react with a base in the dissolved aqueous phase, e.g. sulfite ion, or with a solid phase base, e.g. MgSO.sub.3.3H.sub.2 O. The dissolved aqueous phase base, e.g. sulfite ion, is much more readily available and hence is much more desirable.
In several regenerable processes known in the art, sulfur dioxide is removed from the sulfur dioxide-containing gas stream by contacting the gas with an aqueous solution or slurry in which the following chemical reaction takes place: EQU SO.sub.2 +SO.sub.3.sup.-2 +H.sub.2 O.fwdarw.2HSO.sub.3.sup.-1
Regeneration of the sulfite solution or slurry from the bisulfite is effected as described below. The desirability of such processes depends upon the efficiency with which the sulfur dioxide is trapped in the sorption reaction, which in turn depends on the sulfite concentration, and the ease of regenerating the contacting medium. The result of inefficiency in either reaction is that more of the scrubbing medium must be handled, or more energy must be supplied, thereby increasing the cost of sulfur dioxide removal--larger and longer lines, bigger tanks, pumps, higher temperatures, more electricity, etc.
In processes employing the aforesaid chemistry it is often desirable to remove hydrogen chloride and particulate matter, such as fly ash, from the gas stream before contacting it and to purge sulfate, an oxidation product, from the contacting medium. The presence of chloride and sulfate anions in the contacting medium decreases the amount of sulfite anion which may be present. Since the gas stream is humidified in the process, it is necessary to add makeup water to the contacting medium from time to time to compensate for losses.
One of the regenerable processes of the prior art employs a homogeneous potassium (more recently sodium) sulfite solution as the contacting medium. The Wellman-Lord process, described, for example, in Riesenfeld and Kohl, loc. cit., pp 313-315, is potentially very efficient in the sorption reaction, since sodium and potassium sulfite are quite soluble in water. However, regeneration is accomplished by heating the spent bisulfite-containing solution to drive off the sulfur dioxide, and the regeneration is inefficient. The result is that the Wellman-Lord contacting medium generally contains only about 0.25 gram-mole/liter sulfite.
In variations of the Wellman-Lord process, e.g., the double alkali process (see, for example, Journal of the Air Pollution Control Association, 27, 958 (1977)), the sulfite contacting medium is regenerated by treating the spent bisulfite-containing solution with lime, but the resulting calcium sulfite and calcium sulfate constitute solid byproducts requiring disposal.
Another process well known in the art employs a slurry of magnesium sulfite as the contacting medium, regeneration being effected by treating the spent bisulfite-containing slurry with magnesium oxide or hydroxide (see, e.g., U.S. Pat. No. 3,577,219; U.S. Pat. No. 3,617,212; U.S. Pat. No. 3,653,823; and U.S. Pat. No. 3,826,812). Part of the bisulfite is regenerated to soluble sulfite. The remainder is precipitated as magnesium sulfite solid. Some of the magnesium sulfite solid is separated and may be dried and calcined (see U.S. Pat. No. 3,681,020) to magnesium oxide, which is recycled to the process, and sulfur dioxide, which may be used to make sulfuric acid, etc. The regeneration in the magnesium oxide process is an attractive feature, but the sorption reaction is inefficient. In the first place, magnesium sulfite is not very soluble, and the aqueous scrubbing medium contains only about 0.05-0.2 gram-mole/liter sulfite in solution. Inefficiency in the sorption reaction means that large quantities of the slurry are required. In addition, circulating a slurry causes wear on equipment through erosion, scaling, clogged orifices, etc.
None of the commercial regenerable processes is efficient in both the sorption and regeneration processes, and because of the large volumes of gas generated, requiring treatment with huge volumes of contacting medium, none of the processes of the prior art is adaptable to removing sulfur dioxide from power plant flue gas economically.
Thus, it is an object of this invention to provide a regenerable sulfite sorption process which removes sulfur dioxide effectively, efficiently and economically from a sulfur dioxide-containing gas stream. It is a further objective to provide, within the context of the aforesaid process, a regeneration which is efficient in converting bisulfite to sulfite, while producing an essentially sulfate-free magnesium sulfite, which can be economically transformed into magnesium oxide for recycling. Other advantages will be apparent to those skilled in the art to whom this application is directed.